With the increase in the amount of transmission information, optical interconnect lines in addition to electrical interconnect lines have been used in recent electronic devices and the like. A large number of opto-electric hybrid boards capable of transmitting electrical signals and optical signals at the same time have been used. As shown in FIG. 13, a known example of such opto-electric hybrid boards has a structure in which an electric circuit board E includes an insulation layer 1 made of polyimide and the like and serving as a substrate, and electrical interconnect lines 2 having an electrically conductive pattern and provided on the front surface of the insulation layer 1, and in which an optical waveguide W is provided on the back surface side of the insulation layer 1, with a metal layer 9 for reinforcement provided therebetween (see PTL 1, for example). The front surface of the electric circuit board E is insulated and protected by a coverlay 3. The metal layer 9 is provided with through holes 5 and 5′ for optical coupling between the optical waveguide W and an optical element (not shown) to be mounted on the front surface side of the electric circuit board E. The optical waveguide W includes three layers: an under cladding layer 6; a core 7 serving as an optical path; and an over cladding layer 8.
There is a difference in coefficient of linear expansion between the insulation layer 1 and the optical waveguide W provided on the back surface side thereof. If the insulation layer 1 and the optical waveguide W are directly stacked together, the difference in coefficient of linear expansion therebetween causes stresses and slight bending in the optical waveguide W due to ambient temperature, resulting in increased light propagation losses. The metal layer 9 is provided to avoid such increased light propagation losses. In accordance with trends toward a decrease in the size of electronic devices and an increase in the degree of integration thereof, the opto-electric hybrid boards have been often required to have flexibility in recent years for use in small spaces and in movable sections such as hinges. For the increase in flexibility of an opto-electric hybrid board in which the metal layer 9 is interposed as described above for the provision of the optical waveguide W, it has been proposed to partially remove the metal layer 9 itself to cause the cladding layers of the optical waveguide W to enter the sites where the metal layer 9 is removed, thereby increasing the flexibility (see PTL 2, for example).